Observations from an autonomous underwater vehicle revealed a submarine ridge rising 300m above the sea floor. Until a few decades ago, the glacier was scraping across this underwater ridge, which slowed its flow. However, it has recently thinned and disconnected from the ridge, so that the ice moves more quickly from the land into the sea.

To make matters worse, it also means that relatively warm seawater canflow over the ridge and into a widening inner cavity, more than doubling the ice shelf area exposed to the ocean.

"The discovery of the ridge has raised new questions about whether the current loss of ice from Pine Island Glacier is caused by recent climate change or is a continuation of a longer-term process that began when the glacier disconnected from the ridge," said Dr Adrian Jenkins of the British Antarctic Survey.

The retreat from the ridge started some time before 1970. Since detailed observations of Pine Island Glacier only began in the 1990s, the team now needs to use other techniques such as ice core analysis and computer modelling to try and understand whether this is part of a long term trend.

"Since our first measurements in the Amundsen Sea, estimates of Antarctica’s recent contributions to sea level rise have changed from near-zero to significant and increasing," said Stan Jacobs of the Lamont-Doherty Earth Observatory.

"Now finding that the PIG’s grounding line has recently retreated more than 30 km from a shallow ridge into deeper water, where it is pursued by a warming ocean, only adds to our concern that this region is indeed the 'weak underbelly' of the West Antarctic Ice Sheet."